1. Field of the Invention
The invention relates in general to a local oscillation generator, an associated communication system and a method for local oscillation generation, and more particularly, to a local oscillation generator capable of preventing an amplifier of a communication system from interfering a local oscillator, an associated communication system and a method for local oscillation generation.
2. Description of the Related Art
Communication networks in various standards, such as WLAN, Bluetooth, Mobile Communications, satellite positioning and digital televisions, are an essential part of the modern information society. A communication network includes a transmission end and a reception end. To fully utilize communication channels as well as performing multiplexed accesses, the transmission end and the reception end are provided with respective local oscillation generators for generating local oscillation signals. For example, in a radio-frequency (RF) wireless communication network, the transmission end first encodes information to be transmitted to the communication network into a baseband signal, and a local oscillation signal provided by the local oscillation generator at the transmission end serves as a modulation carrier. By mixing the baseband signal with the carrier, the baseband signal can be modulated and up-converted to an RF signal. A power amplifier at the transmission end further amplifies the power of the RF signal so as to transmit the RF signal to a communication network medium, such as air. Correspondingly, after receiving the RF signal via the communication network medium at the reception end, a local oscillation signal generated by the local oscillation generator of the reception end serves as a demodulation carrier. By mixing the RF signal with the demodulation signal, the RF signal can be demodulated and down-converted to a baseband signal to restore original information before the transmission.
In a local oscillation generator of the prior art, an original oscillation signal is generated by an oscillator, and the original oscillation signal is frequency-divided by an integer to generate a local oscillation signal. For example, assuming a local oscillation frequency required by the local oscillation signal is 2.4 GHz, in such prior art, an original oscillation signal in a 4.8 GHz or 9.6 GHz frequency is generated by the oscillator and is then frequency-divided by 2 or 4 to obtain the 2.4 GHz oscillation signal. On the other hand, due to non-linearity of circuits at the reception end or the transmission end, the local oscillation signal may trigger multiple-frequency harmonic oscillation signals, whose frequency, e.g., 4.8 GHz or 9.6 GHz, is an integral multiple of the local oscillation frequency. However, in such prior art, as the original oscillation frequency based on which the oscillator operates is also an integral multiple of the local oscillation frequency, the multiple-frequency harmonic oscillation signals of the local oscillation signal impose an undesirable effect on operations of the oscillator to thus degrade the communication quality.
For example, at the transmission end, the power amplifier amplifies the power of the RF signal, with the associated non-linearity causing high-power multiple-frequency harmonic oscillation signals. Assuming that the original oscillation frequency that the oscillator utilizes for providing the original oscillation signal is also an integral multiple of the local oscillation signal in a way that a certain multiple-frequency harmonic oscillation signal is extremely approximate to the original oscillation frequency, the frequency of the multiple-frequency harmonic oscillation signals of the power amplifier then causes a frequency pulling effect upon the oscillator. Consequently, an error vector magnitude (EVM) of the transmission end is deteriorated to lead to reduced signal quality. Such occurrence is particularly severe in 2G/3G/4G mobile communication networks and WLAN, as the power amplifier operates at a higher power such as 15 to 30 dBm in the transmission end of these wireless networks.